The present invention relates to a mechanical device which ensures irreversibility in a limit switch assembly for electric actuators coupled through a reversible kinematic chain.
Electric actuators or motors of the type utilized for operating valves are equipped generally with electrical control units by which the movement of the actuator is arrested in positions corresponding to limit positions of the valve. The arrest position is triggered in most instances by a plurality of limit switches, preferably in the form of microswitches, which are operated by moving elements such as flexible tongues, which are independent of each other. The moving elements in their turn are operated by one or more cams carried by a shaft that is connected to a mechanical linkage forming part of the actuator system. The requisite torque of an electric actuator is generated by a motor, a speed reducer, and a worm gear driving a related wheel by means of which motion is transmitted to the valve.
In transmission of the torque, a movement produced by the worm gear acting against a related spring brings about the rotation of a shaft, the shaft in turn bringing about the operation of the limit switches by moving trigger elements mentioned above. In irreversibly coupled systems, on arrival, the worm gear remains in position at the limit even though the motor ceases to produce torque, and returns only when rotated in the opposite direction; thus, the limiter stays engaged.
On the other hand, in reversible systems the worm gear will bring about the operation of the microswitches and duly stop the motor. However the worm gear can then return under the influence of the spring, thereby releasing the switches and allowing the motor to restart if it is still able to do so. The result is that the motor is taken through a succession of stops and starts, giving rise to an undesirable phenomenon referred to as hunting or "pumping" by those skilled in the art.
Such repeated stopping and starting will ultimately damage the mechanical parts associated with the motor.
Hitherto the problem of hunting or pumping has been corrected by means of electrical type of devices which, however, are somewhat costly and which require a small control unit associated with the actuator. Moreover, such devices are often sold separately and must therefore be installed by the purchaser.
The object of the present invention is to overcome the drawbacks aforementioned, and in particular to provide a mechanical device suitable for emplacement directly into a limiter assembly and able to prevent the occurrence of hunting or pumping.
A further object of the invention is to render the embodiment of such a device extremely simple and economic.